Objectives We studied the growth-promoting characteristics of Pseudomonas fluorescens, and the growth promoting effect of P. fluorescens from soil microecosystem, to provide theoretical basis for the application of microbials for sustainable orcharding.

Methods The growth-promoting characteristics of P. fluorescens were identified using different solid culture media and liquid fermentation. A pilot pot experiment was carried out using one-year-old potted peach seedlings, and the suitable concentration of P. fluorescens suspension was found to be 4×10⁸ CFU/mL. Then, a pot experiment was carried out using grafted seedlings of 2-year old ‘Ruiguang39/Prunus’ P. persica (L.) Batsch as experimental material, with 4×10⁸ CFU/mL of P. fluorescens suspension as the treatment (PF), and no P. fluorescens (CK) as the control. On the 7^th, 14^th, 28^th, and 42^nd days of treatment, soil samples around root zone of peach seedlings were collected for the determination of microbial structure, enzyme activity, nutrient content, and pH. On the 20^th and 40^th days of treatment, shoot and root samples were collected for the determination of root growth, shoot nutrient content, leaves photosynthetic characteristics and plant growth of peach.

Results 1) P. fluorescens was proven having the following growth-promoting characteristics: producing 6.17 mg/L of indole-3-acetic acid (IAA), dissolving inorganic and organic P of 45.98 and 18.52 mg/L, respectively, and the capacity of producing siderophores and releasing slowly-available K. 2) Compared with CK, PF treatment increased the Chao 1 and Shannon index of bacteria and fungus communities, and the increment in the Chao 1 and Shannon index of fungus community were as high as 28.5% and 8.5% (P<0.05); PF treatment altered the soil microbial community structure. The relative abundance of Proteobacteria, Actinobacteria, and Acidobacteria were increased, while Bacteroidetes population was reduced. The relative abundance of Ascomycota and Basidiomycetes were decreased, while that of Moridomycetes was increased. The soil pH was decreased by 0.19, available P, N, K and Fe content in PF treatment was significantly increased by 42.3%, 15.9%, 39.5% and 6.6% (P<0.05), and the activities of soil urease, sucrase, phosphatase and catalase were improved by 4.3%, 37.6%, 34.2% and 40.2% (P<0.05), compared to the soil of CK. PF application significantly increased root surface area and volume by 67.3% and 21.3% (P<0.05), and increased the lateral root number. PF treatment significantly increased the net photosynthesis, the contents of total NPK in leaves and roots, and the content of total iron in leaves of peach by 26.0%, thus, significantly increasing the plant height, above- and below ground dry weight by 14.8%, 19.3 % and 19.9% (P<0.05).

Conclusions P. fluorescens has certain growth-promoting characteristics of IAA and siderophores production, degradation of inorganic phosphorus, organic phosphorus and insoluble potassium. Soil application of P. fluorescens suspension increases soil microbial community diversity, alters soil microbial community structure, reduces soil pH, increases peach rhizospheric soil nutrient content and soil enzyme activities. P. fluorescens increases the lateral root number, improves the root architecture, increases the photosynthetic rate, thus increasing the nutrient content and the biomass, thereby promoting the growth of peach.